The present invention is directed generally to systems and methods for filtering and/or delaying transmission signals, and, more particularly, to systems and methods for providing integrated filters with a flat delay response.
The popularity of cellular phones and other wireless communication equipment over the last several years has resulted in the need to obtain greater utilization from the existing frequency spectrum while at the same time reducing the cost of technology used to operate at high frequencies. An important component of any cellular system is the high power amplifier that amplifies signals associated with the cellular system. A particular effective high power amplifier is a feed forward design that typically includes one or more delay lines along a main path and one or more delay lines along a secondary feed forward path.
FIG. 1 shows an exemplary feed-forward high power amplifier 1. In this embodiment, the main path includes a hybrid splitter 2, a phase and amplitude adjustment 3, a main amplifier 4, an output coupler 5, a delay line 6, and an error injection coupler 7, feeding output 13. The secondary path in the embodiment shown in FIG. 1 includes a delay line 8, a hybrid combiner 9, phase and amplitude adjustment circuits 10, 11, and an error amplifier 12. The high power amplifier shown in FIG. 1 is suitable for use as a multi-carrier power amplifier.
The delay lines ideally have a uniform, temperature stable, and fixed amount of insertion delay and constant phase over a predetermined frequency range. Conventionally, the delay lines 6 along the main path are implemented using multi-path delay equalization cavity filters. The multi-path delay equalization cavity filters are configured to utilize cross coupling techniques available due to the greater isolation of cavity filters to minimize delay variations across the passband while handling very high levels of power, e.g., in excess of 1,000 watts for most designs, and over 2,000 watts on a custom basis, even for small cavity diameters. Examples of this type of delay line are available from the assignee of the present application as a standard product line. Filters of this type are typically large measuring from around 15 in2 to more than 40 in2.
Other cavity filter delay lines are, for example, shown in U.S. Pat. Nos. 4,622,523 and 3,699,480. Again, these cavity filters are typically used on the main path of the power amplifier shown in FIG. 1.
On the secondary xe2x80x9cfeed forwardxe2x80x9d path, the conventional implementation of the delay lines (e.g., delay line 8 in FIG. 1) is a coiled coaxial cable or micro-strip printed on a high dielectric material. Examples of devices falling within this category are shown in U.S. Pat. Nos. 4,409,568, 5,252,934, and 4,218,664 and in conventional components such as Murata LDH33, LDH36, and LDH46 series delay lines. Conventional printed micro-strip delay lines, however, are disadvantageous in that there is coupling between the various turns across the high dielectric material and a high insertion loss. Coaxial cable type delay lines have little cross coupling, but still are characterized by high insertion loss, and require a large amount of area to implement. Accordingly, a suitable secondary delay circuit is not now available conventionally.
Various attempts have been made to achieve delay equalization using active components to shift various delay response curves and add them together. For example, U.S. Pat. No. 3,942,118, issued to Haruo Shiki suggests cascading together a first frequency converter, a first filter, a first series of all-frequency pass filters having successively rising resonant frequencies and successively lowering Q-values (first delay equalizer), a second frequency converter, with a second filter circuit, a local oscillator, and a second series of all-frequency pass filters having successively rising resonant frequencies and successively lowering Q-values (second delay equalizer). This circuit, however, has the disadvantage in that there are large insertion losses, large volume resulting from the large number of components, and temperature instability due to the number of active components. Other attempts at active delay equalization, such as, U.S. Pat. No. 4,491,808, suffer from similar problems.
In 1964, Dr. S. B. Cohn proposed using a four-port coupler or a three-port circulator to achieve equalization of non-linear phase angle or time delay characteristics of other components. See, for example, U.S. Pat. No. 3,277,403, herein incorporated by reference. U.S. Pat. Nos. 4,197,514 and 4,988,962 cite examples of Dr. Cohn""s earlier work as prior art in the background portions of the respective specifications. Over the years, there have been several attempts at implementing the structures suggested by S. B. Cohn through the use of bulky, costly, and large devices such as that shown by the above-mentioned U.S. Pat. No. 3,699,480 showing a cavity filter circulator coupled to an impedance circuit. Heretofore, none of these devices has been practical for low cost applications such as the secondary delay line in the multi-carrier power amplifiers discussed in connection with FIG. 1.
Attempts to configure miniaturized implementations using the same designs employed in delay equalized cavity filters have thus far proved unsuccessful due to the cross coupling between the various lumped components of the filter. Heretofore, it was not thought possible to implement such a filter using discrete components.
The lack of shielding in the lumped elements necessarily influences and has a deleterious effect on the overall operation of delay equalization implementations using lumped elements. Conventionally, it was thought that cross coupling in this circuit would result in a device that is nonfunctional for its intended purpose. In the secondary loop, heretofore, it has been thought to be impossible to construct a flat delay response using lumped components configured in a miniaturized configuration because of the numerous stray inductances and the lack of shielding between resonators.
Because of these problems, the present state of the art is to use a coaxial delay line in the secondary path of the feed forward power amplifier. The coaxial delay line in the secondary path, however, has many of the problems discussed above and thus is not satisfactory. Accordingly, the present invention seeks to take an altogether new approach to equalizing the delay across the passband using lumped discrete components. The present invention further seeks to provide improved high and low power delay lines in the main and secondary paths, respectively.
One goal of one or more aspects of the present invention is a delay circuit that is suitable for the secondary path of a feed-forward high power amplifier. According to one embodiment of the invention, such a delay circuit includes a first and a second 3 dB quadrature hybrid coupler that are each preferably miniaturized. The first 3 dB quadrature hybrid coupler has a first node that receives an input signal, a second node that is coupled to a first resonator circuit, a third node that is coupled to a second resonator circuit and a fourth node that is an output node. The first and second resonators respectively include a first and second varactor circuit, and are each resonant at a first predetermined resonant frequency that is a function of a first control voltage that is applied to each of the first and second varactor circuits. The delay response of the first 3 dB quadrature hybrid coupler as a function of frequency is substantially linearly decreasing for increasing frequency greater than the first resonant frequency. The second 3 dB quadrature hybrid coupler has a first node coupled to the fourth node of the first 3 dB quadrature hybrid coupler, a second node that is coupled to a third resonator circuit, a third node that is coupled to a fourth resonator circuit, and a fourth node that is an output node. The third and fourth resonators resonator circuits respectively include a third and a fourth varactor circuit, and are each resonant at a second predetermined resonant frequency that is a function of a second control voltage that is applied to each of the third and fourth varactor circuits. The second predetermined resonant frequency is greater than the first predetermined resonant frequency. The delay response of the second 3 dB quadrature hybrid coupler as a function a frequency is substantially linearly decreasing for increasing frequency less than the second resonant frequency. The overall combined delay response of the first 3 dB quadrature hybrid coupler and the second 3 dB quadrature hybrid coupler appearing at the fourth node of the second 3 dB quadrature hybrid coupler is substantially constant between the first resonant frequency and the second resonant frequency.
Another goal of one or more aspects of the invention is to define a lumped element delay line that has a flat delay characteristic over the passband in a similar manner to how the cavity delay line has a relatively flat delay characteristic over the passband. The present invention utilizes a new configuration for lumped elements previously thought not to be practical in a miniaturized structure. The new architecture utilizes conventional theories to provide a highly improved structure for delay equalization using lumped components to form a flat delay response. This is particularly useful in the secondary feed forward path of the linear amplifier.
One or more aspects of the present invention may solve one or more of the above problems and/or provide improved techniques for implementing delay lines using lumped components.
In one aspect of the invention, a miniaturized delay line assembly is configured using lumped components comprising a miniaturized 3 dB quadrature hybrid coupler (having resonators using lumped components) coupled to two ports and connected in series with a discrete implementation of a band pass filter.
In another aspect of the invention, the lumped components are tuned and mounted in a metal canister prior to shipping to a customer in order to provide the proper equalization. Proper equalization is difficult to achieve using the tolerances normally associated with printed circuit boards assembled on a production basis. In this manner, the delay line may be provided as a pre-assembled part to various manufacturers, such as for use in the secondary path of power amplifiers.
In still another aspect of the invention, a miniaturized four port hybrid quadrature coupler is configured with discrete L-C resonators as the lumped components at two ports coupled in series with a filter, preferably a bandpass filter itself formed from lumped components. In still further aspects of the invention, the miniaturized 3 dB quadrature hybrid coupler is mounted to the same circuit board as the filter with the resonators mounted in close proximity to the miniaturized 3 dB quadrature hybrid coupler, e.g., within xc2xd inch and more preferably within xc2xc inch.
In still further aspects of the invention, the resonator components are discrete components mounted on the miniaturized 3 dB quadrature hybrid coupler. In yet further aspects of the invention, the resonator components are mounted on top of the miniaturized 3 dB quadrature hybrid coupler.
In still further embodiments of the invention, the filter is implemented using miniature ceramic resonators such as the KEL-FIL(copyright) class of ceramic filters.
In yet further aspects of the invention, there is described a method of manufacturing the delay lines such that pre-tested filters are distributed to the field.
In other aspects of the invention, multiply delay circuits are included along with a filter circuit.
In still more aspects of the invention, the delay gain of the circuit is split between the delay circuits and the filter circuits such that the delay circuits comprise 60% or less of the total delay and preferably 50% or less of the total delay and most preferably about 40% of the total delay.
Further aspects of the invention provide for:
In a cavity filter having a plurality of resonators, a capacitive probe providing coupling between a pair of resonators having an odd number of resonators in a signal path there between. The pair of resonators may preferably include a resonator disposed at a turn in the signal path.
A cavity delay arrangement utilizing mixed capacitive and inductive coupling between at least some of a plurality of resonators.
A high power stripline or microstrip three dB hybrid quadrature 90-degree phase shift multicoupler loaded by resonators. An all-pole Chebychev filter may be cascaded with the high power multicoupler.
A pre-tuned delay line module. The module may be used as either a main delay line or a secondary delay line in a feed-forward high-power amplifier.
These and other features of the invention will be apparent upon consideration of the following detailed description of preferred embodiments. Although the invention has been defined using the appended claims, these claims are exemplary in that one or more aspects of the invention are intended to include the elements and steps described herein in any combination or subcombination. For example, it is intended that each of the above aspects of the invention may be used individually and/or in combination with one or more other aspects of the invention defined above and/or in connection with the detailed description below. Accordingly, there are any number of alternative combinations for defining the invention, which incorporate one or more elements from the specification, including the description, claims, aspects of the invention, and/or drawings, in various combinations or subcombinations. Moreover, it will be apparent to those skilled in microwave theory and design, in light of the present specification, that alternate combinations and subcombinations of one or more aspects of the present invention, either alone or in combination with one or more elements and/or steps defined herein, may constitute alternate aspects of the invention. It is intended that the written description of the invention contained herein cover all such modifications and alterations.